The purpose of this study is to compare the accuracy of the: 1) SpHb RevF sensor (currently marketed), 2) the SpHb RevG sensor (a new, limited release version), and 3) the Pronto-7 sensor (currently marketed version) in patients undergoing spine surgery. The results from the three sensors will be compared to a blood hemoglobin level taken intermittently (as a function of the patient's anesthesia care) and analyzed in the UCSF Clinical Laboratories.